Wood is treated in sulfate cooking process by white liquor containing NaOH and Na.sub.2 S, whereby lignin dissolves and cellulose fibers are released. The mixture of cellulose fibers (pulp) and cooking chemicals is then treated with water, resulting in the generation of black liquor. The black liquor is next concentrated by evaporation in an evaporation plant. The black liquor thereby concentrated is combusted in a soda recovery boiler, and the chemical melt thereby produced and containing mainly Na.sub.2 S and Na.sub.2 CO.sub.3, is dissolved in water, resulting in the generation of green liquor. The green liquor is next causticized by caustic lime (CaO) to white liquor containing NaOH, and lime sludge mainly comprising CaCO.sub.3. This white liquor is transferred back to the digester, and the lime sludge is calcinated in a lime sludge reburning kiln to caustic lime which is recycled back into the causticizing stage.
In a sulfate cellulose mill sulfur emissions are generated mainly in the soda recovery boiler, the evaporation plant and the digester house. In order to decrease the environmental impact of the cellulose mill the sulfur emissions should be minimized. It has been found that an increase in the dry solids content of black liquor causes a decrease in the sulfur emissions of the soda recovery boiler. On the other hand, the sulfur content of green liquor increases as a consequence thereof and thus also results in an increase in the sulfidity of the white liquor as well as the sulfur content of the black liquor. As a result, the overall sulfur emissions of an evaporation plant will increase as a consequence of the continuous increase of sulfur in the black liquor as described above. The instant invention is directed to overcoming this problem by removing the sulfur containing gas during the evaporation process but prior to the last stage thereof.
Finnish published application 75615 (U.S. Pat. No. 4,929,307) shows that the viscosity of the black liquor can be decreased by heating the liquor to a temperature higher than its cooking temperature. Consequently, it is possible to evaporate the black liquor to a higher dry solids content, while also decreasing the sulfur emissions of the soda recovery boiler. Also U.S. Pat. No. 2,711,430 discloses that heating of black liquor causes the release of organic sulfur compounds.
Surprisingly, it has been discovered that the above mentioned phenomena can be utilized in a completely new manner. It is thus an object of the instant invention to utilize the above-mentioned phenomena to control the sulfidity of a sulfate cellulose mill.